Process for the production of a waveguide beam converter

ABSTRACT

Process for the production of a waveguide beam converter for shaping a laser beam collection. A plurality of waveguides are produced and arranged in such a way that at least one individual laser beam can be injected into each waveguide. The waveguides are firstly produced on a substrate using planar technology, and subsequently detached from the substrate over a part of their length, starting from their beam exit ends. The free ends are then arranged and fixed in accordance with an intended output beam arrangement of the output laser beam collection.

BACKGROUND OF THE INVENTION

FIELD OF THE INVENTION

The invention relates to a process for the production of a waveguidebeam converter according to the preamble of claim The process accordingto the invention is used, in particular, for the production of awaveguide beam converter for increasing the radiation density of a laserbeam collection emitted by a linear array of laser diodes.

Individual semiconductor laser diodes are known to represent radiationsources with high radiation density. For physical reasons, however,substantial limitations are found for semiconductor-laser radiationsources with high radiation power. This is because laser diodes outputindividual laser beams in the form of stripes, so that in order toproduce a compact laser beam collection with high radiation power, thestriped individual laser beams need to be arranged above one another.However, the high power loss from semiconductor laser diodes places alimitation on the packing density of individual laser diodes. It istherefore necessary for the individual laser beams emitted by aplurality of individual laser diodes to be concentrated, for exampleusing a waveguide beam converter.

A waveguide beam converter of this type is, for example, disclosed byinternational patent application WO 94/152 34. This document describes afiber-optic arrangement made up of a plurality of curved rectangularoptical fibers, by means of which an input laser beam collection whichis emitted by a linear array of laser diodes and consists of individuallaser beams arranged next to one another, is converted into an outputlaser beam connection made up of striped individual laser beams arrangedabove one another. At their input, the optical fibers are arranged nextto one another in a line, so that each individual laser beam of theinput laser beam collection is injected into a separate optical fiber.At their output, the optical fibers are arranged above one another inthe form of a stack, so that an output laser beam collection ofrectangular cross section is emitted from the end faces of the opticalfibers. This output laser beam collection is subsequently injectedthrough a spherical lens into a fiber laser of circular cross section.

At their input, the optical fibers are fastened in precision grooves inan alignment block, these being designed in such a way that thearrangement and spacing of the optical fibers with respect to oneanother corresponds to the arrangement and spacing of the individuallaser beams injected. The optical fibers are made of silicate glassescontaining alkali metals or alkaline earth metals (soft glass), forexample BAK 5 for the fiber core and BAK 2 for the fiber cladding, whichis in turn enclosed by a supporting clad made, for example, of LAKN 12.The fiber-optic bundle is produced by the following steps:

Firstly, the fiber core with the fiber cladding is produced.Subsequently, the fiber cladding is covered with the material of thesupport clad, which has a significantly higher etching rate than thematerial of the fiber cladding. Before the material of the support cladis subsequently etched off down to a thin support clad layer, theabove-described composite made up of the fiber core, fiber cladding andsupport clad layer is drawn in such a way that, after drawing, the crosssection of the fiber core is still somewhat greater than the intendedfinal size.

A number of optical fibers produced in this way are then stacked on oneanother and connected to one another. The stack is then once moreprovided with a jacket and then again drawn until the stack has thedesired geometry. The optical fibers in the stack are subsequentlyseparated at one end of the stack by etching, fastened next to oneanother in the precision grooves of the alignment block and formed insuch a way that individual laser beams emitted from the linear array oflaser diodes can be injected into the ends of the fibers.

As can be seen from the description above, the production of a waveguidebeam converter of the type mentioned in the introduction is currentlyelaborate and complicated, and therefore very cost-intensive.

SUMMARY OF THE INVENTION

The object of the invention is to provide a process for the productionof a waveguide beam converter which is simple to carry out and makes itpossible, in a straightforward way, to produce a plurality of waveguidebeam converters at the same time.

This object is achieved by a process of the type mentioned in theintroduction, having the features of claim 1.

The advantage of this process according to the invention consists, inparticular, in that the production of the waveguide beam converterrequires only a small number of process steps, known from planartechnology, which are straightforward to carry out.

Further, in the process according to the invention, it is advantageouslypossible to produce glasses of high optical quality on a substrate inaccordance with requirements. The glass compositions may be tailored tothe specific purpose. Radiation losses in the waveguide beam convertercan in this way be kept as small as possible.

Further, it is possible with the process according to the invention, ina straightforward way, to tailor the glass properties to the constraintspertaining to the system as a whole, by varying the glass compositions.

A particular advantage of this production process consists in that thelaser diode array, for example a linear array of laser diodes or aplurality of individual laser diode chips, can be fitted on the samesubstrate as the waveguide beam converter. Likewise, coupling lenses forinjecting the input laser beam collection into the waveguide beamconverter and/or coupling lenses for injecting the output laser beamcollection into an individual optical fiber, a further laser or adifferent device, may also be arranged on this substrate. The resultachieved by this is that both the laser diode array and coupling lenses,which may be necessary, can be mounted in a straightforward way so as tobe precisely aligned with the waveguide beam converter. To this end, forexample, positioning holes or edges may be produced in or on thesubstrate.

It is further advantageous that the waveguides can be made from an SiO₂glass which contains more than 50 cation % of SiO₂. Glasses of this typehave high optical quality. The radiation losses in the waveguide beamconverter, that is to say both in the light-carrying waveguide core andat the interface between the waveguide core and the waveguide cladding,can consequently be kept low. An essential advantage of theaforementioned SiO₂ glasses is that they can be produced by vapordeposition, which substantially facilitates the production of glasslayers on a substrate. Individual glass layers can be produced withoutgreat difficulty free of bubbles and free of cords so that they areplaced precisely above one another, which produces interfaces which arefree of defects and therefore of high optical quality. A furtheradvantage is that these glasses can be removed again by vapor-phaseetching methods.

An advantageous process for the production of a waveguide on asubstrate, the waveguide being detached from the substrate at least overa first part of its length and being connected to the substrate over asecond part of its length, and an advantageous embodiment of the processaccording to the invention are given in claim 4 and claim 5,respectively. An advantage in this case is that use is exclusively madeof process steps known from planar technology, which can be carried outin conventional semiconductor technology fabrication lines.

It is also advantageously possible with the process according to theinvention, in a straightforward way, to produce a waveguide beamconverter in which the waveguides are arranged in such a way that aplurality of output laser beam collections can be extracted from them.As a result, it is advantageously possible for the input laser beamcollection emitted by a laser diode array to be shaped into a pluralityof rectangular output laser beam collections, which can then in turn beinjected very efficiently into waveguides with a round cross section.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic perspective representation of a waveguide beamconverter which is produced using the process according to theinvention,

FIG. 2 is a schematic representation of a side view of a waveguide beamconverter which is produced using the process according to the inventionand which, at its input, is optically coupled at its input to a laserdiode array and, at its output, to an optical fiber,

FIG. 3 shows a schematic representation of a plan view of a secondembodiment of a waveguide beam converter, which is produced using theprocess according to the invention,

FIG. 4 shows a schematic representation of the process according to theinvention as it is carried out.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the arrangement represented in FIG. 1, a waveguide beam converter 2is arranged on a substrate 1, for example a silicon substrate, theconverter consisting, for example, of 5 waveguides 3 to 7. At the beaminput ends 8, the waveguides 3 to 7 are arranged lying next to oneanother on the substrate 1. The waveguides 3-7 are detached from thesubstrate over a portion 27 of their length and, at the beam exit ends9, the waveguides 3-7 are combined and fixed to form a stack 10consisting of waveguide ends arranged above one another.

The individual laser beams which are emitted by a laser diode array 11,for example a linear array of laser diodes, and each have, for example,a striped cross section, can be injected into the beam entry ends 8 ofthe waveguides 3 to 7. The individual laser beams may consist of asingle laser beam or of a plurality of individual laser beams which aresmaller in cross section. An individual laser beam which is striped incross section may have a beam width of about 200 μμm and a beam heightof about 2 μm.

A beam collimator 12 (FIG. 2) for example a cylindrical lens or adiffraction grating, which at least reduces any vertical divergence ofthe individual laser beams, may be arranged between the laser diodearray 11 and the light entry ends 8. The dimensions of the waveguides 3to 7 are latched to the dimensions of the individual laser beams to beinjected.

A rectangular output laser beam collection can be extracted from thebeam exit ends 9 arranged as a stack 10, and can be injected, forexample, through a spherical lens or directly into a waveguide 14, forexample circular in cross section, into a fiber laser or into anotherlaser.

As shown in FIG. 3, the process according to the invention may also beused to produce waveguide beam converters 2 with which an input laserbeam collection made up of a plurality of individual laser beams can beshaped into a plurality of output laser beam collections, eachconsisting of a number of individual laser beams. In this embodiment,the ends of the waveguide bundles are, for example, fitted into sleeves25, and this is naturally also possible in all the other embodiments.

The waveguide beam converter 2 may, naturally, both in the embodimentrepresented in FIG. 1 and in the one represented in FIG. 3, be designedfor an arbitrary number of individual laser beams. It is merelynecessary to match the number of waveguides to the number of individuallaser beams.

The process according to the invention is generally carried out asfollows:

Firstly, a number of waveguides 3-7 (for example consisting of SiO₂glass with ≧50 cation % SiO₂) are produced in the form of an array oflinear stripes, on a substrate, for example a silicon substrate, usingplanar technology methods. The cross section, the acceptance angle andthe spacing of the waveguides 3-7 are in this case matched exactly tothe characteristics of the input laser beam collection. The waveguides 3to 7 are then, starting from their beam exit ends 9, detached from thesubstrate 1 over a portion 27 of their length, using a suitable etchingtechnique and when appropriate while preserving their cross-sectionalshape. The free portions 27 of the lengths of the waveguides 3 to 7 arethen shifted, while avoiding too small radii of curvature, out of a beamplane of the input laser beam collection, and are combined to form astack 10. The shape of the cross section of this stack 10 may be arectangle, parallelogram or other shape suitable for the purpose inquestion.

In the detailed procedure of a process according to the invention,schematically represented in FIG. 4, a partition layer 16, for exampleconsisting of pure SiO₂, SiO₂—GeO₂ glass or SiO₂—P₂O₅ glass and having athickness of about 0.75-3 μm, is firstly applied to a (100) siliconwafer 15. Next, a first cladding glass layer 17 is applied to thepartition layer 16, and a core layer 18 is applied to this claddingglass layer 17. The first cladding glass layer 17 consists, for example,of SiO₂ glass with ≧50 cation % SiO₂, the remaining cations beingpreferably selected from the group B, Ge, P, Ti. A glass having thefollowing composition is, in particular, suitable: 20-35 cation %BO_(1.5)/0.7-1.2 cation % GeO₂/64-78 cation % SiO₂.

The first cladding glass layer 17 has, for example, a thickness of about5 to 10 μm. The core layer 18 consists, for example, likewise of SiO₂glass with ≧50 cation % SiO₂, the remaining cations being preferablyselected from the group B, Ge, P, Ti, but the Ge and/or Ti content beingconsiderably higher than in the cladding glass layer 17. A glass havingthe following composition is, in particular, suitable: 15-20 cation %BO_(1.5)/8-18 cation % GeO₂/60-75 cation % SiO₂. By increasing the Geand/or Ti content, a higher refractive index is obtained in comparisonwith the first cladding glass layer 17, corresponding to the desirednumerical aperture. The core layer 18 is then structured usingphotolithography in conjunction with dry and/or wet etching. The width,length and spacing of the waveguide cores 18 are thereby set relative toone another. These values are matched to the dimensions of the laserdiode array 11 and the characteristics of the input laser beamcollection.

A second cladding glass layer 21, which consists of SiO₂ glass with ≧50cation % SiO₂, the remaining cations preferably being selected from thegroup B, Ge, P, Ti, is then applied to the waveguide cores 19 remainingon the first cladding glass layer 17 and to the free surfaces 20 of thecladding glass layer 17. A glass having the following composition is, inparticular, suitable: 20-35 cation % BO_(1.5)/0.7-1.2 cation %GeO₂/64-78 cation % SiO₂. The second cladding glass layer 21 has, forexample, a thickness of from 5 to 10 μm.

An example of a suitable production process for individual glass layersis flame hydrolysis, a method known from fiberoptic technology, which isin particular suitable for the production of glass layers (thickness ≧5μm). It is also, however, possible to use another process known by theperson skilled in the art to be suitable.

Before the waveguides are etched free, a part of the cladding glasslayers 17, 21 lying between the wave guide cores 19, as well as thepartition layer 16, are removed in such a way that a number of waveguidecores 19 fully enclosed by a cladding glass layer 17, 21 is left, andthe silicon wafer 15 is exposed between them.

The waveguides are detached over a part of their length in the followingway:

Optionally, a part of the partition layer 16 lying between the firstcladding glass layer 17 and the (100) silicon wafer 15 is removed fromboth sides of the waveguides 22, for example by etching with an NH₄F/HFetch mixture. This has been found to be suitable for initiating theetching of the (100) silicon wafer 15. It has also been found favorableto select the composition of the partition layer 16 in such a way thatit can be etched more quickly than the neighboring materials, or atleast more quickly than the first cladding glass layer 17 which liesabove the partition layer 16. This is, for example, achieved in that theglass layers of the waveguides consist of B₂O₃—SiO₂ glass whichdissolves more slowly in an NH₄/HF etch mixture than B₂O₃-free SiO₂glass. The rule found is: the higher the B content, the lower theetching rate.

By partially etching away the partition layer 16 in this way, the upperside of the silicon wafer is partially exposed below the waveguides 22.This step of etching the partition may be repeated several times whilethe (100) silicon is being etched away below the waveguides 22, and inalternation with this etching. Etching away the partition layer 16 canprevent regions with a lower etching rate, for example (111) faces,which may occur in the form of peaks under the glass front, from causingdetrimental retention of the etching front below the waveguides.

After the partition layer 16 has been etched away, the (100) silicon isthen removed using a suitable etchant (for example KOH solution, about50% by weight, 50° C.) below the waveguides 22 in such a way that thewaveguides 22 are detached from the silicon wafer 15 over a part oftheir length (selective etching with an etching ratiov((100)-Si):v(B-doped SiO₂ glass)≈80:1). The waveguides 22 are arrangedparallel to the (100) direction on the surface of the silicon wafer 15,so that a U-shaped trench 26 can be formed between the waveguides andthe silicon wafer 15. This trench 26 ideally has exactly verticalbounding faces.

Instead of a (100) silicon wafer 15, it is for example also possible touse a (110) silicon wafer or a different semiconductor wafer. Theorientation of the waveguides 22, the compositions of the individualglass layers and the etchants must then, naturally, be tailored to therelevant situation.

A critical point in the production process is represented by the tensionstate of the partition layer 16. If the partition layer 16 is undertensile stress relative to the glass of the waveguides 22 (the partitionlayer has a higher coefficient of thermal expansion than the overlyingcladding glass layer (17), then this may lead to fracture when thesubstrate is detached. If it is under compressive stress (for examplepartition layer made of pure SiO₂), then this risk is less. A partitionlayer 16 of this type must be thick enough so that cracks in it producedduring the detachment do not penetrate the tensioned glass layers of thewaveguides 22, which may also cause damage to the waveguides 22. A 0.75μm-3 μm thick layer of SiO₂—Ge₂ glass or SiO₂—P₂O₅ glass meets theaforementioned requirements. It may either be applied directly to thesubstrate by flame hydrolysis or approximated by a 2-layer system of 200μm thermal SiO₂ followed by 1-3 μm SiO₂—GeO₂ glass.

As a further possibility, a graded layer may also be formed as thepartition layer 16. In this case, a thin (for example 750 μm thick) Sioxide layer is firstly grown. A first cladding glass layer 17, to whichGeO₂ is added, is then applied to the oxide layer. During a heattreatment in the further course of the process, for example at 1200° C.,Ge diffuses into the SiO₂ layer. The partition layer 16 is used upduring this procedure or removed after the KOH etching in an NH₄F/HFcleaning etching step. The partition layer 16 is preserved over thelength of the waveguides 22 which is not etched free.

To further optimize the stress pattern in the waveguides 22 and toreduce the risk of fracture, the composition may be controlled in thethree-phase system SiO₂—B₂O₃—GeO₂ in such a way that, while maintainingthe desired refractive indices of the core glass layer 18 and of thecladding glass layers 17, 21, the coefficient of thermal expansionincreases from the first cladding glass layer 17, through the core glasslayer 18, to the second cladding glass layer 21. TiO₂ or P₂O₅ mayoptionally be added to the aforementioned three-phase system. Theexpansion coefficient of the second cladding glass layer 21 is thenclosest to that of the silicon wafer 15. This can be done, in the caseof refractive-index compensation using GeO₂, through an increase in theB₂O₃ content or the P₂O₅ content. Waveguides 22 produced in this wayhave, when etched free, a specific upward curvature. During theconcluding phase of the process of detaching the waveguides 22, it istherefore less likely that cracks will cause accidental fracture.

Since, after a sufficient time of etching back in KOH and/or NH₄/HF,cracks caused by the detachment are rounded off or removed, a sufficientfinal strength of the waveguides 22 can be obtained without furthermeasures being taken. Nevertheless, to round the edges further, asubsequent heat treatment may also be carried out at temperatures ofabout 800° C.

To secure and bond/solder the beam exit ends of the waveguides 22, thatis to say, for example, of the fiber stack(s) 24, and to finish the beamexit ends, use may be made of an L- or U-shaped or closed sleeve 25 intowhich the exposed upwardly curved waveguides 22 are placed, for exampleinserted.

The beam exit ends of the waveguides 22 may be sawed and polished orprovided with an antireflection layer and bloomed. This is also true ofthe beam entry ends of the waveguides 22.

In order to avoid aging effects (stress-crack corrosion), it is possiblefor the waveguides 22 to have a protective layer applied to them, forexample made of a glass having a high SiO₂ content or of silicon nitride(this can be produced by plasma methods) and having a thickness of about2 μm.

Beside the function mentioned above, the partition layer 16 has thepurpose of protecting the Si crystal against the diffusion of impuritiesinto it, which can cause a change in the rate at which the silicon isetched. Boron diffusion from the glass layers arranged above (claddingglass layers 17, 21 and core layer 18) leads, for example, to a drasticreduction in the Si etching rate in KOH, so that detachment of thewaveguides 22 is practically prevented. The partition layer 16 musttherefore, at least at the boundary with the silicon wafer, be free ofdopants that reduce the silicon etching rate.

With the process described above, it is advantageously possible, byvarying the glass compositions, to tailor the glass properties to theconstraints given for the system as a whole. The layer thicknesses andthe numerical aperture of the waveguides can be matched quickly in theproduction process to the input laser beam collection.

A further advantage of the process according to the invention consistsin the fact that the spacings between the waveguides on the substratecan be matched by photolithographic methods straightforwardly to therepeat interval of the laser diode array.

What is claimed is:
 1. A process for producing a waveguide beamconverter for geometrical shaping laser beams, which comprises:producing waveguides each having a beam exit end and a given length on asubstrate using planar technology; configuring and disposing each of thewaveguides for receiving at least one individual input laser beam;detaching at least one of the waveguides, over a part of the givenlength, from the substrate starting from the beam exit end; and formingand fastening the waveguides for producing a desired output beam patternof a collection of output laser beams derived from a collection of inputlaser beams.
 2. The process according to claim 1, which comprisesfitting a laser diode array having a plurality of individual laser beamsoutputting the collection of input laser beams for forming thecollection of output laser beams on the substrate.
 3. The processaccording to claim 1, which comprises fitting a number of couplinglenses for forming the collection of input laser beams on the substrate.4. The process according to claim 1, which comprises fitting a laserdiode array having a plurality of individual laser beams and a number ofcoupling lenses for outputting and forming the collection of input laserbeams forming the collection of output laser beams on the substrate. 5.The process according to claim 1, which comprises producing thewaveguides from a glass containing more than 50 cation % of SiO₂.
 6. Aprocess for producing at least one waveguide on a substrate, the atleast one waveguide having a length and being detached from thesubstrate at least over a first part of the length and being connectedto the substrate over a second part of the length, the first part of theat least one waveguide being moveable relative to the second part, whichcomprises: providing a substrate; applying a partition layer to thesubstrate; applying a first cladding glass layer to the partition layer;applying a core layer to the first cladding glass layer and structuringa waveguide core from the core layer; applying a second cladding glasslayer to the waveguide core and to free subregions formed in the corelayer next to the waveguide core for fully enclosing the waveguide coreby the first cladding glass layer and the second cladding glass layerfor forming a waveguide; and detaching the waveguide from the substrateover a first part of a length of the waveguide.
 7. A process forproducing a waveguide beam converter for geometrical shaping laserbeams, which comprises: providing a substrate; applying a partitionlayer to the substrate; applying a first cladding glass layer to thepartition layer; applying a core layer to the first cladding glasslayer; structuring the core layer for forming a number of waveguidecores separated from one another and creating free subregions in thecore layer, the waveguide cores remaining on the first cladding glasslayer; applying a second cladding glass layer to the waveguide cores andto the free subregions between the waveguide cores for fully enclosingthe waveguide cores by the first cladding glass layer and the secondcladding glass layer; removing parts of the first cladding glass layerand the second cladding glass layers lying between the waveguide coresforming a number of waveguides separated from one another; removing thepartition layer over a first part of a length of the waveguides fordetaching the waveguides from the substrate over the first part of thelength; configuring and disposing each of the waveguides for receivingat least one individual input laser beam; and forming and fastening thewaveguides for producing a desired output beam pattern of a collectionof output laser beams derived from a collection of input laser beams.